The present technology relates to electrolytic capacitors, and more particularly to methods of filling porous, hydrophobic separator materials with polar electrolytes.
Most implantable medical devices (IMDs) employ a battery, and in the case of an implantable cardiac defibrillator (ICD), also a capacitor bank including one or more high voltage electrolytic capacitors. These capacitors may each have an operating voltage ranging from about 200 to 400 V. Such operating requirements have generally necessitated use of wet electrolytic technology that in turn requires a mechanical separator system to ensure the separation of electrodes of opposite polarity; i.e., cathode and anode. The separators employed in these electrochemical cells are typically of such construction that they are wettable with whatever liquid electrolyte is used in the electrochemical cell.
In the case of some electrochemical cells, notably advanced valve metal (AVM) capacitors, the liquid electrolyte is comprised in part of a very polar constituent that causes the electrolyte to exhibit a high wetting angle on relatively hydrophobic materials often used as separators. For example, if the polar liquid electrolyte is attracted to the separator surface, such as water on a strongly hydrophilic surface, the droplet will spread out on the solid surface and the contact angle will be close to 0°. Hydrophilic separator materials are hence readily wetted by polar electrolytes. Conversely, a less hydrophilic separator will have a contact angle up to 90°, and highly hydrophobic surfaces may have contact angles as high as 150°, or even nearly 180°. On these hydrophobic surfaces, polar electrolyte droplets simply rest on the surface, without actually wetting the separator material to any significant extent. These surfaces are termed super-hydrophobic, and include for example, fluorinated materials (e.g., Teflon® and Teflon®-like coatings) that may be micro-patterned and/or that are micro-porous.
A substrate material for use in an electrochemical cell is preferably chemically inert and structurally stable. Materials exhibiting these properties include hydrophobic materials having low surface energies, such as the aforementioned fluorinated materials. While such materials may be advantageously nonreactive with components of the polar electrolyte, such as acids and/or various solvents, these materials are nevertheless difficult to wet with a polar electrolyte. Wetting of a hydrophobic substrate with a polar electrolyte has been facilitated by using one or more surfactants and/or surface energy modifications of the substrate material. However, alternative approaches that promote impregnation of polar electrolyte into porous, hydrophobic separator materials are desirable.